Reservoir storage is divided into four zones, or pools. These include, from top to bottom, the flood-control zone, conservation zone, buffer zone and inactive zone. The conservation and buffer pools, together, constitute the reservoir's active storage. WEAP will ensure that the flood-control zone is always kept vacant, i.e., the volume of water in the reservoir cannot exceed the top of the conservation pool.
WEAP allows the reservoir to freely release water from the conservation pool to fully meet withdrawal and other downstream requirements, and demand for energy from hydropower. Once the storage level drops into the buffer pool, the release will be restricted according to the buffer coefficient, to conserve the reservoir's dwindling supplies. Water in the inactive pool is not available for allocation, although under extreme conditions evaporation may draw the reservoir into the inactive pool.
To define the zones, you enter the volumes corresponding to the top of each zone (Top of Conservation, Top of Buffer and Top of Inactive). WEAP uses the Buffer Coefficient to slow releases when the storage level falls into the buffer zone. When this occurs, the monthly release cannot exceed the volume of water in the buffer zone multiplied by this coefficient. In other words, the buffer coefficient is the fraction of the water in the buffer zone available each month for release. Thus, a coefficient close to 1.0 will cause demands to be met more fully while rapidly emptying the buffer zone, while a coefficient close to 0 will leave demands unmet while preserving the storage in the buffer zone. Essentially, the top of buffer should represent the volume at which releases are to be cut back, and the buffer coefficient determines the amount of the cut back. Note: The buffer coefficient determines how much of the water that is in the buffer zone at the beginning of a timestep is available for release during that timestep. However, this doesn't restrict WEAP from releasing some or all of water that flows into the reservoir during the timestep. Even if the buffer coefficient is 0, WEAP can still release any water that flows into the reservoir that timestep if needed to meet downstream or hydropower demands--in this case, the storage level will not decrease, but it may not increase either.
See River Reservoir Flows for calculation algorithms.
Entered on: 
Data View, Branch: Supply
and Resources \ Local or River \ Reservoir, Category: Operation, Tabs:
Top of Conservation, Top of Buffer, Top of Inactive, Buffer Coefficient